win-dshow: Use AVFrame.pts instead of AVFrame pkt_pts

AVFrame.pkt_pts was deprecated in FFmpeg commit 32c8359093d1 dated June
21, 2016 for lavc 57.24.0 (FFmpeg 3.2). AVFrame.pkt_pts was replaced by
AVFrame.pts, and it was removed in FFmpeg commit 6e30b35b85b8 [2] on
April 27, 2021. Ubuntu 18.04 has the oldest FFmpeg version we support,
FFmpeg 3.4. We should be able to safely migrate from AVFrame.pkt_pts to
AVFrame.pts.

[1]: https://github.com/FFmpeg/FFmpeg/commit/32c8359093d1ff4f45ed19518b449b3ac3769d27
[2]: https://github.com/FFmpeg/FFmpeg/commit/6e30b35b85b81c802e52a1078ec7a3097e353c6d